The main components of the outer face of the outer membrane of Salmonella typhimurium are lipopolysaccharide (LPS) and specific outer-membrane proteins. Most "rfa" genes, determining the structure of the LPS core, are located between "cysE" and "pyrE," including several determining FOR character, that is core defect permitting normal addition of O chains. The order of such genes will be investigated by transduction using phage ES18.h1 and deletion mapping: complementation of "rfa" mutants and possible "rfa" operons will be tested, using an F' factor. Already identified genes determining outer membrane proteins will be mapped by transduction and/or conjugation. New phages and bacteriocins using outer membrane proteins for adsorption will be sought, to facilitate isolation and mapping of new types of outer membrane protein mutant, including those deficient in uptake of iron chelates. The genetic fine structure of gene "chr" will be investigated. The effect on virulence of "chr" and other mutations affecting iron uptake will be examined. Mutations causing complete blocks in synthesis of metabolites essential for bacterial growth but absent (or nearly so) from mammalian tissues will be sought, in virulent S. typhimurium, and the effect of such mutations on virulence determined. Smooth strains, non-virulent because of stable auxotrophic mutations, will be tested as live vaccines, and the degree, specificity and persistence of induced immunity determined. Improved methods for study of in vitro phagoctosis of Salmonella and of the fate of phagocytosed bacteria (death, survival, multiplication) will be used to investigate the mechanism of the loss of virulence resulting from alterations in LPS structure or nutritional character. The mode of action of newly discovered bacteriocins (colicins of group E and a bacteriocin active on rough Salmmonella) will be examined. The possible effect of somme plasmids on outer membrane composition, LPS structure and antibiotic sensitivity of rough mutants of Salmonella will be examined.